The present invention relates to a projection display apparatus and, more particularly, to a projection display apparatus using a spatial light modulation device which utilizes a diffraction effect and the spatial light modulation device applied to the apparatus.
Projection display apparatuses using a liquid crystal panel as a spatial light modulation device allow display with a screen size of 100 to 200 inches and have widely been put into practice as display apparatuses for TV images or computers. When a liquid crystal panel is used for a projection display apparatus instead of a CRT, a compact and lightweight apparatus with low power consumption and low cost can be realized.
The display apparatus using a liquid crystal panel has such various advantages, though it also has a disadvantage that the utilization efficiency of light from the light source is as low as several %, and the displayed image is dark. When this display apparatus is used to display a TV image for the purpose of, e.g., showing a motion picture, this problem is not so serious because it can be solved by darkening the room. However, a data projector for displaying a computer screen is often used in a bright place, and therefore it is essential to display a bright image.
In the projection display apparatus using a liquid crystal panel, the decrease in light utilization efficiency is mainly caused by the fact that the half of light incident on the liquid crystal panel is not used because polarization is used for light modulation. To realize display of a bright image, modulation techniques without using polarization have been studied. One of such techniques uses a spatial light modulation device (called a diffraction type spatial light modulation device) which utilizes a diffraction effect.
FIG. 1 shows the schematic arrangement of an optical system in a projection display apparatus using a diffraction type spatial light modulation device. This optical system basically illuminates a diffraction type spatial light modulation device 105 with light from a light source 101, and projects an image on the spatial light modulation device 105 onto a screen 109 through a projecting lens 106. More specifically, in the illumination optical system, white light emitted from the light source 101 is converged by a condenser lens 102. The opening is limited by a light source-side aperture stop 103 arranged at the converging position. Thereafter, the light is collimated into an almost parallel beam by a collimator lens 104 and irradiated on the spatial light modulation device 105.
The light which irradiates the spatial light modulation device 105 is limited in its maximum incident angle (or spread angle) with respect to the spatial light modulation device 105 by the actions of the light source-side aperture stop 103 and the collimator lens 104, i.e., increased in its directivity. In the spatial light modulation device 105, the incident light directly passes through some pixels, but is subjected in some pixels to complex amplitude modulation having a spatial periodical structure and diffracted in a predetermined direction.
In the projection side optical system, the image on the spatial light modulation device 105 is projected onto the screen 109 through the projecting lens 106. In this optical system, of light components which are not diffracted by the pixels of the spatial light modulation device 105, only light components reflected or absorbed by a light-shielding plate 107 and diffracted reach the screen 109. Consequently, a difference is generated in the brightness of the projected image between the pixels in which the light is diffracted and those in which the light is not diffracted, so that display with a contrast is enabled.
As the diffraction type spatial light modulation device 105, a device having a structure in which a liquid crystal is sandwiched between a first substrate having a striped transparent electrode and a second substrate having a transparent electrode formed on the entire surface is used, as described in, e.g., the report by Y. Hori et al., (IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol. ED-26, No. 11 (1979): to be referred to as reference 1 hereinafter). When no voltage is applied between the transparent electrodes on the two substrates, the liquid crystal molecules are uniformly aligned so that the device optically seems a flat plate. When a voltage is applied, the alignment state of the liquid crystal changes along the striped transparent electrode, and the effect of a diffraction grating is obtained. Therefore, by turning on/off the applied voltage, white/black display is enabled. With the device structure shown in reference 1, the diffraction effect is expected to depend on the polarization direction of incident light. However, when the electrode arrangement is improved, the alignment state of the liquid crystal may be controlled, and the dependency on the polarization may be minimized. This may realize bright display.
To display a bright and high-contrast image on the screen 109 in the projection display apparatus using the diffraction type spatial light modulation device 105, a small and high-luminance light source may be used as the light source 101, and the size of the light-shielding plate 107 on the projection side may be optimized in correspondence with the size of the light source 101 or the size of the light source-side aperture stop 103.
Actually, however, the light-emitting point of a light source with a sufficient luminance has a predetermined size or more. Additionally, to efficiently pass the light from the light source 101, the light source-side aperture stop 103 cannot be extremely minimized. When the size of the light-shielding plate 107 on the projection side is set in correspondence with these members, the brightness and contrast of the image displayed on the screen 109 cannot be simultaneously increased. More specifically, to maintain the brightness of the projection display apparatus using not a diffraction type but a normal spatial light modulation device, the contrast is insufficient, i.e., only about 10 to 20, according to reference 1.
The optical system shown in FIG. 1 projects light diffracted by the spatial light modulation device 105 onto the screen 109 and is called a dark field projection optical system. In this dark field projection optical system, a black image can be sufficiently darkened, so that the display contrast can be made high. The projection display apparatus using a diffraction type spatial light modulation device generally uses this system.
In this dark field projection optical system, display of a white image corresponds to diffraction in pixels of the spatial light modulation device 105, as described above. For this reason, the light utilization efficiency in display of a white image largely depends on the shape of the diffraction grating on the spatial light modulation device 105 or the wavelength bandwidth of the illumination light. To obtain a high efficiency, the diffraction grating shape must be strictly managed. Therefore, a spatial light modulation device for obtaining a high light utilization efficiency can hardly be manufactured. In addition, since the uniformity of brightness of display depends on the uniformity of the diffraction grating shape, display at a high and uniform light utilization efficiency can hardly be realized. Furthermore, since this optical system uses diffracted light for projection, a lens having a small F-number must be used as the projecting lens 106. The projection lens tends to be large, resulting in an increase in cost. A compact and inexpensive apparatus can hardly be manufactured.
To the contrary, an optical system (bright field projection optical system) which has, in place of the light-shielding plate 107 serving as a projection side light-shielding device, an opening limiting device for passing only light near the optical axis, shields diffracted light, and projects only non-diffracted light onto the screen can be used. Since this system uses light passing near the optical axis for projection, the F-number of the projecting lens can be relatively large, so that a small and inexpensive projecting lens can be used. Display of a white image corresponds to pixels which do not cause diffraction. Since the light utilization efficiency in display of a white image does not depend on the shape of the diffraction grating or the wavelength bandwidth, display at a high light utilization efficiency and with uniform brightness can be easily realized. However, when a black image is to be displayed in this system, the contrast becomes low because the black image cannot be sufficiently darkened.
As described above, the diffraction type spatial light modulation device is one of devices capable of optical modulation independently of polarization and allows bright display in principle. However, when the diffraction type spatial light modulation device is incorporated in the optical system of the projection display apparatus and combined with an actually usable light source, the brightness or uniformity and the contrast of an image can hardly be simultaneously increased.
Conventionally, to solve this problem, the relationship between the size of the light source or the opening of the light source-side aperture stop and the size of the light-shielding surface of the light-shielding plate on the projection side is optimized. However, to maintain the advantage of this arrangement, i.e., the brightness of an image in comparison with a projection display apparatus using a normal spatial light modulation device, the contrast is sacrificed.